X-ray tubes can include an electron source, such as a filament, which can emit an electron beam into an evacuated chamber towards an anode target. The electron beam causes the anode target material to emit elemental-specific, characteristic x-rays and Bremsstrahlung x-rays. X-rays emitted from the anode target material can impinge upon a sample. The sample can then emit elemental-specific x-rays. These sample emitted x-rays can be received and analyzed. Because each material emits x-rays that are characteristic of the elements in the material, the elements in the sample material can be identified.
The characteristic x-rays emitted from both the target and the sample can include K-lines and L-lines for K and L electron orbital atomic transitions respectively. The K-lines of a given element are higher in energy than the L-lines for that element. For quantification of the amount of an element in the sample, it is important that a K-line or an L-line in the anode target have a higher energy than a K-line or an L-line in the sample. It is also desirable for the K-line or the L-line in the anode target to have an energy relatively close to the K-line or L-line in the sample, in order to maximize the K-line or L-line x-ray signal from the sample, thus improving the accuracy and precision of analysis.
If an L-line from the x-ray tube's anode target is higher than and close to the energy of a K-line or L-line in the sample, then the anode target L-line can be used for identification and quantification of the elements in the sample and it is desirable that the x-ray tube emit more of the target L-line x-rays and less K-line x-rays. The energy of the electrons impinging the target can be reduced by changing the x-ray tube voltage, thus causing the target to emit more L-line x-rays and less or no K-line x-rays. Thus the x-ray tube can emit relatively more L-line x-rays and less K-line and Bremsstrahlung x-rays. If the electron energy, controlled by the tube voltage, is lower than the energy of the K-line of the target, the K-line will not be emitted.
If a K-line from the x-ray tube's anode target is higher and close to the energy of a K-line or L-line in the sample, then the anode target K-line can be used for identification and quantification of the material in the sample and it is desirable that the x-ray tube emit more of the target K-line x-rays. The x-ray tube voltage can be increased in order to cause the x-ray tube to emit relatively more K-line x-rays. Thus it is desirable to adjust the x-ray tube voltage depending on the material that is being analyzed.
In a transmission x-ray tube, the use of a single anode target for multiple x-ray tube voltages can result in non-optimal use of the electron beam. A higher tube voltage can produce a higher energy electron beam. A higher energy electron beam can penetrate deeper into an anode target material. If the target material is too thin, then some of the electrons pass through the anode target material. Electrons that pass through the target anode material do not result in x-ray production by the target material and the overall efficiency of the electron to x-ray conversion is reduced. This is detrimental to the analysis of the sample since a higher rate of x-ray production can improve the precision and accuracy of analysis and reduces the time of measurement.
A lower tube voltage can produce a lower energy electron beam. A lower energy electron beam will not penetrate as deeply into the target material as will a higher energy beam. If the target material is too thick, then some of the x-rays produced will be absorbed by the target anode material. Target absorbed x-rays are not emitted towards the sample. This is another inefficient use of the electron beam.
Inefficient use of the electron beam to create the desired x-rays is undesirable because a longer sampling time is then required for material analysis than if all the electrons were used for production of target emitted x-rays. Thus if the target anode material is optimized for use at high x-ray tube voltages, then when used at low x-ray tube voltages, some of the target x-rays will be absorbed by the target material. If the target material is optimized for use at low x-ray tube voltages, then when used at high x-ray tube voltages, some of the electron beam will pass through the target material without production of x-rays.
If the target material target is compromised at an intermediate thickness, then at low tube voltage, some target produced x-rays will be reabsorbed by the target material, but not as many as if the target material was optimized for high tube voltage. Also, at high tube voltage, some of the electron beam will pass through the target, but not as much as if the target material was optimized for low tube voltage. Thus there is a problem at both high and low tube voltages.
Multiple targets may be used for production of different wavelengths of x-rays. For example, see U.S. Pat. Nos. 4,870,671; 4,007,375, and Japanese Patent Nos. JP 5-135722 and JP 4-171700. One target may be optimized for one tube voltage and another target may be optimized for a different tube voltage. A problem with multiple targets can be that the x-rays emitted from one target can be directed to a different location than x-rays emitted from a different target. This can create problems for the user who may then need to realign the x-ray tube or tube optics each time a transition is made from one target to another target.
The need to realign the x-ray tube or tube optics may be overcome by use of a layered target, with each layer comprised of a different material. For example, see U.S. Pat. No. 7,203,283. A problem with a layered target can be that an x-ray spectrum emitted from a layered target can contain energy lines originating from all target layers making the analysis more cumbersome and less precise.
X-rays emitted from multiple targets can be directed by optics towards the sample material. For example, see U.S. Patent Publication No. 2007/0165780 and WIPO Publication No. WO 2008/052002. Additional optics can have the disadvantage of increased complexity and cost.